Question

I am goofing around with pointers and structures. I want to achieve the following:
(1) define a linked list with a structure (numberRecord)
(2) write a function that fills a linked list with some sample records by going thourgh a loop (fillList)
(3) count the number of elements in the linked list
(4) print the number of elements

I am now so far that the fillList function works well, but I do not succeed in handing over the filled linked list to a pointer in the main(). In the code below, the printList function only displays the single record that was added in main() instead of displaying the list that was created in the function fillList.

---

#include <stdio.h>
#include <stdlib.h>

typedef struct numberRecord numberRecord;

//linked list
struct numberRecord {
             int number;
        struct numberRecord *next;
};

//count #records in linked list
int countList(struct numberRecord *record) {

         struct numberRecord *index = record;
    int i = 0;

    if (record == NULL)
        return i;

    while (index->next != NULL) {
        ++i;
        index = index->next;
    }

    return i + 1;
}

//print linked list
void printList (struct numberRecord *record) {

    struct numberRecord *index = record;

    if (index == NULL)
        printf("List is empty \n");

    while (index != NULL) {

        printf("%i \n", index->number);
        index = index->next;
    }

}

//fill the linked list with some sample records
void fillList(numberRecord *record) {

    numberRecord *first, *prev, *new, *buffer;

//as soon as you add more records you get an memory error, static construction
    new = (numberRecord *)malloc(100 * sizeof(numberRecord));
    new->number = 0;
    new->next = NULL;

    first = new;
    prev = new;
    buffer = new;

    int i;

    for (i = 1; i < 11; i++) {

        new++;

        new->number = i;
        new->next = NULL;

        prev->next = new;
        prev = prev->next;
    }

    record = first;
}


int main(void) {

    numberRecord *list;
    list = malloc(sizeof(numberRecord));
    list->number = 1;
    list->next = NULL;

    fillList(list);
    printf("ListCount: %i \n", countList(list));
    printList(list);
    return 0;
}

SOLUTION
Do read the posts below, they indicated this solution and contain some very insightful remarks about pointers. Below the adapted code that works:

#include <stdio.h>
#include <stdlib.h>

typedef struct numberRecord numberRecord;

//linked list
struct numberRecord {
             int number;
        struct numberRecord *next;
};

//count #records in linked list
int countList(struct numberRecord *record) {

         struct numberRecord *index = record;
    int i = 0;

    if (record == NULL)
        return i;

    while (index->next != NULL) {
        ++i;
        index = index->next;
    }

    return i + 1;
}

//print linked list
void printList (struct numberRecord *record) {

    struct numberRecord *index = record;

    if (index == NULL)
        printf("List is empty \n");

    while (index != NULL) {

        printf("%i \n", index->number);
        index = index->next;
    }

}

//fill the linked list with some sample records
 numberRecord *fillList() {

    numberRecord *firstRec, *prevRec, *newRec;

    int i;

    for (i = 1; i < 11; i++) {

        newRec = malloc(sizeof(numberRecord));
        newRec->number = i;
        newRec->next = NULL;

        //initialize firstRec and prevRec with newRec, firstRec remains head
        if (i == 1) {
            firstRec = newRec;
            prevRec = newRec;
        }
        prevRec->next = newRec;
        prevRec = prevRec->next;
    }

    return firstRec;
}


int main(void) {

    numberRecord *list;
    list = fillList();

    printf("ListCount: %i \n", countList(list));
    printList(list);
    return 0;
}

Answer 1

This statement in fillList

record = first;

has no effect on the list variable in main. Pointers are passed by value (like everything else) in C. If you want to update the list variable in main, you’ll either have to pass a pointer to it (&list) and modify fillList accordingly, or return a numberRecord* from fillList. (I’d actually go with that second option.)

Here’s a (bad) illustration:

When main calls fillList, at the starting point of that function, the pointers are like this:

main        memory       fillList
list ----> 0x01234 <----  record

A bit later in fillList, you allocate some storage for new (that’s actually a bad name, it conflicts with an operator in C++, will get people confused)

main        memory       fillList
list ----> 0x01234 <----  record
           0x03123 <----  new

At the last line of fillList you’re left with:

main        memory       fillList
list ----> 0x01234   ,--  record
           0x03123 <----  new

record and list are not the same variable. They start out with the same value, but changing record will not change list. The fact that they are both pointers doesn’t make them any different from say ints in this respect.

You can change the thing pointed to by list in fillList, but you can’t change what list points to (with your version of the code).

The easiest way for you to get around that is to change fillList like this:

numberRecord *fillList() {
  ....
  return new;
}

And in main, don’t allocate list directly, just call fillList() to initialize it.